Coating composition

ABSTRACT

A Chromium VI-free coating composition comprises a binder and a corrosion inhibitor in a solvent, wherein the binder comprises a silicate and an organic titanate, and the corrosion inhibitor comprises aluminium particles and zinc particles preferably flakes. The composition may also comprise an anti-corrosion additive which comprises a metal phosphate, a lubricant which comprises a polyolefin wax, and/or a thickener. The coating composition is useful for coating substrates such as nuts, bolts and other fasteners, door, bonnet and boot lock parts, hinges, door stoppers, window guides, seat belt components, brake rotors and drums, and other transportation industry related parts.

FIELD OF THE INVENTION

[0001] The present invention relates to a coating composition, for usein the protection of metal surfaces, which comprises a binder, acorrosion inhibitor, and a solvent.

BACKGROUND OF THE INVENTION

[0002] It is well known to treat metal surfaces such as iron and steelwith some form of corrosion inhibiting treatment or coating. Corrosioninhibiting coatings are well known in the art and generally containmetal particles in particular zinc and/or aluminium particles as activeingredients together with some form of binder.

[0003] GB 1380748 describes a coating composition particularly, but notexclusively, for a zinc filled coating composition which when applied toa metal surface will provide galvanic protection to the metal. Thecomposition comprises trialkoxysilanes which have been cohydrolysed andcocondensed with a hydrolysable titanium ester. The Silane is selectedfrom RSi(OR′)₃ and RSi(OR″OR ″)₃ where R and R′ are monovalent aliphaticor aromatic hydrocarbon radicals having up to 10 carbon atoms, R″ is adivalent hydrocarbon radical having from 2 to 6 carbon atoms and R′″ aremonovalent aliphatic or aromatic hydrocarbon radicals having up to 10carbon atoms or hydrogen.

[0004] GB 1499556 relates to a process for hydrolyzing ethyl silicate toform a gellable liquid hydrolysate which is used for mixing with powderssuch as powdered zinc for use in an anti-corrosion paint. The ethylsilicate is acid hydrolysed and the solvent for the hydrolysis isacetone or an alcohol

[0005] EP0808883 discloses a water-reducible coating composition forcorrosion protection comprising particulate metal such as aluminium orzinc and a water reducible organofunctional silane. Epoxy silanes,particularly beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and/orgamma glycidoxypropyltrimethoxysilane were preferred. Other constituentsincluded a high boiling point organic liquid, a water soluble cellulosebased thickener and a wetting agent.

[0006] U.S. Pat. No. 5,393,611 and U.S. Pat. No. 5,324,545 both relateto a dip-coating method for protecting chromatised or passivated zinccoatings on steel or the like using a composition of a titanic acidester and a “so-called” organofunctional polysiloxane, preferably havingbetween 2 and 10 siloxane repeating units and epoxy end groups. There isno clear definition of the meaning of the term organofunctionalpolysiloxane in either of these documents but it would seem to mean apolymer with a siloxane backbone having at least one Si—R bond where Ris an unsaturated or functionally substituted hydrocarbon radical.Confusingly however the examples in U.S. Pat. No. 5,393,611 and U.S.Pat. No. 5,324,545 both teach that rather than an organofunctionalpolysiloxane being used the preferred silicon containing compound is anepoxy silane, namely gamma glycidoxypropyltrimethoxysilane.

[0007] The present inventors have found an increasing demand fromindustry for coatings for metal surfaces which can provide a high levelof corrosion protection, cathodic protection, and “for-life” drylubrication (i.e. the metal surface needs coating only once during itsworking life) with defined and constant coefficient of friction, whilstbeing Chromium VI-free and providing an attractive appearance toarticles coated with the coating. Commercially available coatings areunable to satisfy all of these demands.

BRIEF SUMMARY OF THE INVENTION

[0008] According to the present invention there is provided a coatingcomposition which comprises a binder and a corrosion inhibitor in asolvent, wherein the binder comprises a silicate and an organictitanate, and the corrosion inhibitor comprises aluminium particles andzinc particles.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The binder used in the composition of the present inventioncomprises a silicate and an organic titanate. For the avoidance ofdoubt, it is to be understood that the term silicate is used to mean acompound which contains substantially no Si—C bonds, i.e. that carbonlinkages to silicon in silicates as described in this invention aresubstantially always via an oxygen atom (i.e. an Si—O—C bond). Mostpreferably a silicate in accordance with this invention contains no Si—Cbonds. Preferably, the binder comprises 20 to 60% by weight (e.g. 30 to45%) silicate, and 40 to 80% by weight (e.g. 55 to 70%) organic titanateto a total of 100% by weight.

[0010] Suitable silicates include colloidal silica and organicsilicates, with the latter being preferred. Suitable organic silicatesinclude silicate esters, for example silicate ester monomers (e.g. ethylsilicate), hydrolysate (e.g. silicic ester hydrolysate) andalkoxysilanes, preferably tetraalkoxysilanes, although silicate esterpolymers are most preferred, (e.g. alkyl polysilicates where the alkylgroup has 1 to 6 carbon atoms and is most preferably methyl or ethyl).

[0011] Suitable organic titanates include titanate chelates (e.g.titanium acetylacetonate and triethanolamine titanate) and titanateesters, with the latter being preferred. Suitable titanate estersinclude titanate ester monomers (e.g. tetraalkyltitanates wherein eachalkyl group is the same or different and contains between 1 and 12carbon atoms, examples include tetrabutyltitanate,tetraisooctyltitanate, and tetraisopropyltitanate), although titanateester polymers are preferred (e.g. alkylpolytitanates such asbutylpolytitanate).

[0012] The corrosion inhibitor used in the composition of the presentinvention comprises aluminium particles and zinc particles. Thealuminium particles may be in the form of powder, paste or flake, withaluminium flake (leafing or non-leafing) being preferred. The aluminiumparticles preferably have an average particle size of 4 to 20 μm, morepreferably 6-15 μm. The zinc particles may be in the form of zincpowder, for example zinc spheres or zinc flake, preferably zinc flake.The zinc particles preferably have an average particle size of 6 to 26μm, more preferably 8-15 μm. The corrosion inhibitor preferablycomprises 80 to 97% by weight (e.g. 87 to 95%) of zinc particles, and 3to 20% by weight (e.g. 5 to 13%) of aluminium particles to a total of100% by weight.

[0013] The composition of the present invention may also comprise ametal phosphate as an anti-corrosion additive. Preferred metalphosphates are zinc phosphates, including modified zinc orthophosphates(e.g. modified zinc aluminium-orthophosphatehydrate) and modified zincpolyphosphates (e.g. modified zinc aluminium-polyphosphate hydrate),with the latter being most preferred. The metal phosphate may be presentin an amount of up to 33% by weight of the solid content of thecomposition of the present invention (i.e. without the solvent),preferably 5 to 20% by weight:

[0014] The composition of the present invention may further comprise athickener, e.g. silica and/or organic modified clay, in an amount of upto 4% by weight of the solid content of the composition, preferably from1 to 3% by weight.

[0015] The composition of the present invention may still furthercomprise a lubricant, for example a wax, including hydrocarbon waxes andpolytetrafluoroethylene (PTFE) wax, preferably a polyolefin-containingwax (e.g. micronised polypropylene hydrocarbon wax), in an amount of upto 8% by weight of the solid content of the composition, preferably from1.5 to 4.5% by weight.

[0016] Suitable solvents for use in the composition of the presentinvention are well known in the art. Organic solvents are suitable,including alcohols (e.g. methanol, ethanol, propanol, butanol), ketones(e.g. acetone, methyl ethyl ketone, methyl butyl ketone, cyclohexanone),esters (e.g. butyl acetate), and mixtures thereof. However, preferredsolvents for the coating composition are hydrocarbon solvents, inparticular white spirits, due to their high evaporation rates and lowlevels of aromatic compounds. Particularly preferred white spirits arethose containing C₁₁-C₁₆ normal, iso- and cycloalkanes.

[0017] The coating composition of the present invention thus comprises abinder and corrosion inhibitor in a solvent, and preferably a metalphosphate anti-corrosion additive, a lubricant, and a thickener.Preferably, the solid content of the composition comprises 50 to 80%,more preferably 65 to 80%, by weight corrosion inhibitor, 9 to 18%, morepreferably 11 to 16%, by weight of binder, up to 33%, more preferably 5to 20% by weight of metal phosphate, up to 8%, more preferably 1.5 to4.5% by weight of lubricant, and up to 4%, more preferably 1 to 3% byweight thickener.

[0018] The coating composition of the present invention can be preparedby mixing its components together using conventional apparatus,preferably by first blending some of the solvent and binder, then addingthe corrosion inhibitor, and then finally adding the remaining solvent.

[0019] The coating composition of the present invention may be appliedto a surface by any conventional application technique, for examplebrushing, dip-spinning dipping, and spraying (e.g. by aerosol can).Other common application methods include spraying drums, centrifuges,electrostatic or automatic spraying, printing and roller coating. Thechosen method of application will depend upon the shape, size, weightand quantity of items to be coated. Preferably, 2, 3 or more coatinglayers are applied. The coating thickness has an influence on the lifeand properties of the resulting coating, and should be greater than theroughness of the surface, typically from 5 to 25 μm. Once the surfacehas been coated with the composition, it is dried to evaporate thesolvent and cure the coating. The coating composition can be cured by,for example, heating at 200° C. for 10 minutes.

[0020] The coating composition of the present invention may be usedalone or in combination with other commercially available anti-frictionor anti-corrosive coatings. A preferred commercially availableanti-friction coating which may be utilised in combination with thecomposition of the present invention has the following composition(percentages by weight), and is referred to hereinafter as “top-coat A”:

[0021] 20-25% lubricant—mixture of phenolic, epoxy and vinyl butyralresins, and PTFE.

[0022] 70-75% solvent—mixture of methyl ethyl ketone, methyl isobutylketone, and cyclohexanone.

[0023] The coating composition of the present invention may be used incombination with a metal particle free top-coat hereafter referred to as“top-coat B” comprising a silicate as herein before described and anorganic titanate as herein before described. The top-coat may optionallyinclude any other components as herein before described for the coatingcomposition of the present invention other than the metal (i.e. zinc andaluminium) particles. The top-coat is described in the applicantsco-pending patent application GB 0110627.7.

[0024] Particularly preferred combinations of coating layers include:

[0025] 1, 2 or 3 coating layers of the composition of the presentinvention.

[0026] 1, 2 or 3 coating layers of the composition of the presentinvention followed by 1 to 3 coating layers of top-coat B

[0027] 1 or 2 coating layers of the composition of the present inventionfollowed by 1, 2 or 3 coating layers of top-coat A.

[0028] Another possibility is the combination of all three coatings,i.e. 1 or 2 coating layers of the composition of the present inventionfollowed by 1, 2 or 3 coating layers of top-coat A and finally followedby 1 to 3 coating layers of top-coat B, although this combination ishighly unlikely to be utilised for reasons of cost alone.

[0029] Substrates may be pretreated prior to coating with the coatingcomposition of the present invention to improve adhesion and life of theresulting protective coating. Conventional methods of pretreatmentinclude degreasing (for example, using solvents or steam), treatment ofcorroded surfaces by acid or alkali, phosphating, oxalic acid treatmentof stainless steel, sandblasting and anodizing.

[0030] The coating composition according to the present invention canthus be used to provide a protective coating, for metals prone tocorrosion, such as iron and steel. The provision of a protective coatingon a substrate will result in high corrosion resistance, cathodicprotection, and, when a lubricant is utilised, for life lubrication witha defined and constant coefficient of friction for articles such asautomotive components is provided, for example nuts, bolts and otherfasteners, door, bonnet and boot lock parts, hinges, door stoppers,window guides, seat belt components, brake rotors and drums, and othertransportation industry related parts.

[0031] A further embodiment of the present invention relates to asubstrate coated with the coating composition as hereinbefore describedand to a method of coating such a substrate with a coating compositionas hereinbefore described.

EXAMPLES

[0032] The present invention will now be illustrated by way of example.All percentages are by weight.

Example 1

[0033] A coating composition according to the present invention wasprepared by mixing the following materials:

[0034] 8% ethyl silicate polymer

[0035] 13% polybutyl titanate

[0036] 3% aluminium pigment

[0037] 33% zinc pigment

[0038] 5% zinc-aluminium phosphate

[0039] 34% petroleum white spirit

[0040] 2% polypropylene wax

[0041] 0.6% silica

[0042] 0.6% organic modified clay

Example 2 Substrate Pretreatment

[0043] Steel bolts, 10 mm diameter by 60 mm in length, were pretreatedby sandblasting.

Example 3 Substrate Coating

[0044] The pretreated bolts of Example 2 above were coated with coatingcompositions AF1 to AF3 below. Each coating layer was applied by dipspinning in a centrifuge, partial curing for 10 minutes at 200° C.,followed by further dip spinning and full cure at 200° C. for 10minutes.

[0045] AF1—2 coating composition of Example 1 alone.

[0046] AF2—3 coating composition of Example 1 alone.

[0047] AF3—2 coating composition of Example 1 followed by 2 coatinglayers of Coating A described hereinabove.

[0048] A comparative anti-friction coating (CAFI) was also preparedwhich consists of 3 coating layers of Coating A described hereinabove.

Example 4 Corrosion Resistance

[0049] Salt spray test DIN 50021 was performed on the AF1 to AF3 boltsprepared according to Example 3 above. The results are shown in Tables 1to 3 below (average results taken from test results for 10 bolts): TABLE1 coating AF1 % red rust corrosion Test Time (hours) Thread Head 1 4800.0 0.0 900 0.0 0.0 2 480 0.0 0.0 900 0.0 0.0 3 480 0.0 0.0 900 0.0 0.04 480 0.0 0.0 900 0.0 0.0

[0050] TABLE 2 coating AF2 % red rust corrosion Test Time (hours) ThreadHead 1 480 0.0 0.0 720 0.0 0.0 >900 0.0 0.0 2 480 0.0 0.0 720 0.00.0 >900 0.0 0.0 3 480 0.0 0.0 720 0.0 0.0 >900 0.0 0.0 4 480 0.0 0.0720 0.0 0.0 >900 0.0 0.0

[0051] TABLE 3 coating AF3 % red rust corrosion Test Time (hours) ThreadHead 1 480 0.0 0.0 900 0.0 0.0 2 480 0.0 0.0 900 0.0 0.0 3 480 0.0 0.0900 0.0 0.0 4 480 0.0 0.0 900 0.0 0.0

Example 5 Lubrication

[0052] The coefficient of friction of the AF 1 and AF3 coated boltsprepared according to Example 3 above was determined using an ErichsenAP 541 Bolt Testing Machine. Testing was performed on bolts having beentightened 1 and 3 times and against different surfaces. The results areshown in Table 4 below: TABLE 4 Total coeff. Coating Surface Tighteningsof friction ±variation None Steel 1 0.195 0.035 AF1 Steel 1 0.112 0.003AF1 Steel 1 0.123 0.005 AF1 Painted steel 1 0.129 0.008 AF1 Aluminium 10.138 0.003 AF1 Steel 3 0.114 0.003 AF1 Steel 3 0.127 0.007 AF1 Paintedsteel 3 0.131 0.009 AF1 Aluminium 3 0.140 0.004 AF3 Steel 1 0.114 0.006AF3 Steel 1 0.100 0.006 AF3 Painted steel 1 0.109 0.008 AF3 Aluminium 10.104 0.002 AF3 Steel 3 0.114 0.009 AF3 Steel 3 0.108 0.013 AF3 Paintedsteel 3 0.114 0.012 AF3 Aluminium 3 0.114 0.007

Example 6 Cathodic Protection

[0053] Unpretreated iron panels were degreased and coated on one sidewith the coating composition of Example 1 above and with theanti-friction coating CAF1 using an Erichsen spiral film applicator. Thepanels were then cured for 10 minutes at 200° C. After cooling, the drythickness of the cured coatings was measured. Corrosion protection tapewas then applied to the untreated surfaces of each panel and an X-cutmade in the coated surface of each panel until the metal surface wasreached. The panels were then placed into a salt spray tester (DIN50021) until red rust formation was noticed, and the results are givenin Table 5 below: TABLE 5 red rust formation after AF coating Thickness(μm) [hours] Example 1 4 144 Example 1 7.5 312 CAF1 10 24

Example 7

[0054] Bolts of the type described in example 2 were coated with 2layers of the coating composition of example 1 and cured as described inExample 3. After curing the coated bolts were provided with twoalternate top-coats of the type referred to above as coating B. Thetop-coat compositions were prepared by mixing the materials identifiedin Table 6 as follows:—

[0055] The polybutyl titanate and ethyl polysilicate were added into amixing kettle with a dissolver disk for a period of 10 minutes.Simultaneously a slurry of the silica, clay zinc-aluminium phosphateand, when present, polypropylene wax in a proportion of the Petroleumwhite spirit (about 9% by weight of solvent in sample 2 and about 20% byweight of solvent in example 1) was prepared in an Ultra turraxhomogeniser. The slurry was then added into polybutyl titanate and ethylpolysilicate mixture and the resulting mixture was mixed with thedissolver disk for a period of 30 minutes at which time the residualamount of solvent was added and the final mixture was mixed in thepresence of the dissolver disk for a further 10 minutes. It will be seenthat sample TC1 omits the anti-corrosion additive zinc-aluminiumphosphate. TABLE 6 Sample TC1 Sample TC2 Components Wt % Wt % Petroleumwhite spirit 47.45 43.61 Polybutyl titanate 24.34 22.38 Ethylpolysilicate 24.34 22.38 Silica 1.15 1.06 Organic modified clay 1.000.91 zinc-aluminium phosphate 0.00 8.08 Polypropylene wax 1.72 1.58complete: 100.00 100.00

[0056] The top-coat compositions, samples TC1 and TC2 were applied in anidentical fashion to the coating composition of the present inventionand each layer applied was cured at 200° C. for 10 minutes.

Example 8 Corrosion Resistance

[0057] Salt spray test DIN 50021 was performed on the bolts prepared asdiscussed in Example 7. The results are shown in Table 7 below (averageresults taken from test results for 10 bolts). In each test two layersof the coating composition in accordance with the invention were coatedon to each bolt but the number of layers of the alternative top-coatswas varied as indicated in Table 7: TABLE 7 Corrosion resistance (withtop-coat) % of red rust on head of Coating Time (hours) bolt Coating 8.1900 0.0 (no top-coat) Coating 8.2 2000 1.7 (1 layer of TC1) Coating 8.32000 0.0 (1 layer of TC2) Coating 8.4 2000 1.0 (2 layers of TC1) Coating8.5 2000 0.0 (2 layers of TC2)

[0058] The above should be compared with the results provided in Table7a in which the same test was carried out with a commercially availableproduct which comprises zinc and aluminium particles and a bindercomprising a mixture of tetrabutyltitanate and trimethoxyvinylsilaneboth with and without a top-coat. It is understood that the comparativetop-coat is an organic resin comprising phenolic and epoxy componentswhich may in addition comprise up to about 30% by weight ofpolytetrafluoroethylene (PTFE). It will be noted that the amount of redrust which appears on bolts coated with the comparativebase-coat/comparative top-coat combination is significantly greater thanfor coatings comprising only the comparative base-coat. Furthermore,sets of comparative results shown in Table 7a are also significantlyworse than the results in Tables 1, 2 and 3. TABLE 7a Comparative % ofred rust on head Coatings Time (hours) of bolt Comparative base-coat 2403.0 (2 layers of base-coat only) 480 6.0 Comparative base-coat +top-coat 240 16.0 (2 layers of comp. Base-coat + 1 layer of comptop-coat)

Example 9 Lubrication (with Top-Coat)

[0059] The coefficient of friction of the coated bolts preparedaccording to Example 8 was analysed as described in Example 5. Testingwas performed on bolts having been tightened 1 and 3 times using a steelsurface. Coatings 9.1, 9.2 and 9.3 are equivalent to coatings 8.1, 8.4and 8.5 in example 8. The results are shown in Table 8 below: TABLE 8Lubrication (with top-coat B) Coeff. of Coating Tightenings Surfacefriction ±variation 9.1 1 Steel 0.117 0.005 9.1 3 Steel 0.117 0.004 9.21 Steel 0.122 0.003 9.2 3 Steel 0.124 0.004 9.3 1 Steel 0.127 0.003 5.33 0.118 0.003

1. A coating composition which comprises a binder and a corrosioninhibitor in a solvent, wherein the binder comprises a silicate and anorganic titanate, and the corrosion inhibitor comprises aluminiumparticles and zinc particles.
 2. A composition according to claim 1wherein the binder comprises 20 to 60% by weight silicate and 40 to 80%by weight organic titanate to a total of 100% by weight.
 3. Acomposition according to claim 1 wherein the corrosion inhibitorcomprises 80 to 97% by weight of zinc particles and 3 to 20% by weightof aluminium+particles to a total of 100% by weight.
 4. A compositionaccording to claim 1 which further includes an anti-corrosion additivewhich comprises a metal phosphate.
 5. A composition according to claim 1which further includes a lubricant which is selected from the group of ahydrocarbon or polytetraethylene wax.
 6. A composition according toclaim 1 which further includes a thickener.
 7. A composition accordingto claim 1 having a solid content which comprises 50 to 80% by weight ofcorrosion inhibitor, 9 to 18% by weight of binder, 0 to 33% by weight ofa metal phosphate, 0 to 8% by weight of a lubricant, and 0 to 4% byweight of a thickener.
 8. A substrate having a coating which is formedfrom 1, 2 or 3 coating layer(s) of a composition according to claim 1.9. A substrate having a coating which is formed from 1, 2 or 3 layer(s)of a composition according to claim 1 and a further 1, 2 or 3 coatinglayers of an additional coating composition which comprises 20-25% byweight of a lubricant mixture of phenolic resin, epoxy resin, vinylbutyral resin, and polytetrafluoroethylene, and 70-75% by weight of asolvent mixture of methyl ethyl ketone, methyl isobutyl ketone, andcyclohexanone.
 10. A substrate having a coating in accordance with claim8, wherein there is provided a further 1 to 3 layers of a metal particlefree top-coat comprising a silicate and an organic titanate in asolvent.
 11. A substrate in accordance with any one of claims 8 whereinthe substrate is selected from nuts, bolts and other fasteners, door,bonnet and boot lock parts, hinges, door stoppers, window guides, seatbelt components, brake rotors and drums, and other transportationindustry related parts.
 12. A substrate in accordance with claim 9wherein the substrate is selected from nuts, bolts and other fasteners,door, bonnet and boot lock parts, hinges, door stoppers, window guides,seat belt components, brake rotors and drums, and other transportationindustry related parts.
 13. A process for providing a substrate with aprotective coating comprising applying a composition in accordance withclaim 1 onto the substrate.
 14. A process in accordance with claim 13wherein the coating is formed on the substrate from 1 or 2 coatinglayer(s) of a composition in accordance with claim 1, followed by one orboth of:— (i) a further 1, 2 or 3 coating layers of an additionalcoating composition which comprises 20-25% by weight of a lubricantmixture of phenolic resin, epoxy resin, vinyl butyral resin, andpolytetrafluoroethylene, and 70-75% by weight of a solvent mixture ofmethyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone and (ii)1 to 3 layers of a metal particle free top-coat comprising a silicateand an organic titanate.
 15. A substrate having a protective coatingobtainable by the process in accordance with claim 13.